Eclipse is a platform, and a lot of plugins are there to make it usable for your language(s). Where IDEA is only Java, Eclipse lets me do PHP too. As I do both regularly, it's a compelling "feature".
IDEA lets me do Java, PHP, RoR, html, etc. I'd say it's just as much of a platform as Eclipse is, and has a huge number of plug-ins to extend its feature set.
This is a common misconception. IDEA was once "only Java", but it is rapidly becoming a generalized platform, as well. Writing Ruby with IDEA is an absolute delight, for instance,